Sheet steel with a conversion coating, method of producing conversion-coated sheet steel and treatment agent for application of a conversion coating on sheet steel

ABSTRACT

A sheet steel, in particular strip-shaped blackplate, with a conversion coating which is prepared from components dissolved in water. The components are selected from the group of i) hexafluorotitanate, ii) zinc phosphate, iii) phosphoric acid, iv) a mixture of any of i) to iii), provided that the components i), ii) and iii) do not contain any organic ingredients.

FIELD OF THE DISCLOSURE

The disclosure relates to sheet steel with a conversion coating, amethod of producing conversion-coated sheet steel and a treatment agentfor application of a conversion coating on sheet steel.

BACKGROUND

In the prior art, it is known to protect metal surfaces againstcorrosion by employing methods in which the metal surface is coated witha coating of a different, generally less noble, metal (e.g., zinc andchromium). Thus, it is known, e.g., to coat sheet steel with zinc orchromium or even tin (which is more noble than steel). In the productionof packaging materials, particularly in the food sector, for example,extensively tin-coated blackplate (tinplate) is used. The distinguishingfeatures of tinplate are excellent corrosion resistance, goodformability properties and its weldability, which makes tinplateuniquely well suited for use in the production of packaging materials,e.g., beverage cans.

Tinplate has excellent properties as a packaging material for foodproducts and has been produced and processed for this purpose for manydecades. However, tin, which in tinplate constitutes thecorrosion-inhibiting coating, has become a relatively high-valuematerial because of the increasing worldwide shortage of this resource.As an alternative to tinplate, especially for use as a packagingmaterial, it is known from the prior art to use sheet steel which iselectrolytically coated with chromium and which is called “Tin-FreeSteel (TFS)” or “Electrolytic Chromium Coated Steel (ECCS).” On the onehand, this tin-free sheet steel is distinguished by its excellentadhesion for lacquers or organic protective coatings (for example, PP orPET), on the other hand, however, the coating process entailsconsiderable disadvantages due to the toxic and health-endangeringproperties of the chromium(VI)-containing materials used in the coating.

To protect sheet steel against corrosion and to create a good adhesionsurface for lacquers and synthetic coatings, conversion coatings arefrequently applied to the surface of sheet steel. The targeted creationof a conversion coating on sheet steel (blackplate) prevents thecorrosion of sheet steel or at least slows it down considerably.

Conversion coatings are very thin non-metal coatings on a metal surface,which, as a rule, are created by chemical reaction of an aqueoustreatment solution with the metal substrate.

Especially when applied to thin sheet steel (blackplate with thicknessesin the range of 0.1 to 0.5 mm), conversion coatings provide a highlyeffective protection against corrosion and a good adhesion surface forlacquers and synthetic materials, and they reduce surface friction andabrasion.

Electrolytic methods of applying conversion coatings to sheet steel areknown from the prior art. Such conversion coatings were frequentlycreated with chromium electrolytes based on carcinogenic chromium(VI)oxide. Due to statutory prohibitions, however, the use ofchromium(VI)-containing conversion coatings is becoming increasinglyrare. Alternatives to the classic chromium(VI) electrolyte aretreatments based on chromium(III) oxide or complex fluorides (titaniumcompounds, zirconium compounds). Another possibility of forming aconversion coating is phosphating by means of aqueous phosphatesolutions.

DE 101 61 383 A1 describes a method of coating metal surfaces, includingsteel surfaces, with an aqueous composition free of chromium(VI)compounds, with the aqueous composition, in addition to the solventwater, containing at least one organic film-forming substance with awater-soluble or water-dispersible polymer, a content of cations and/orhexa- and tetrafluoro complexes of cations, selected from the group ofTi, Zr, Hf, Si, Al and B, at least one inorganic compound in the form ofparticles with a particle diameter from 0.005 μm to 0.2 μm, andoptionally a silane and/or siloxane, and optionally a corrosioninhibitor as well.

Conventional, commercially available chromium-free agents for formingconversion coatings on metal contain either film-forming substances ororganic solvents. As a rule, film-forming substances are polymers whichimpart a number of valuable properties, such as adhesive capacity, tothe conversion coating. However, the application of these polymersubstances requires additional labor and equipment, which also makes thesubstances more costly. As a rule, the solvents contained inconventional chromium-free substances used to form conversion coatingsare more expensive than water, they pose some risk to human healthduring the application of the conversion coating, and in most cases,they belong to the class of the so-called VOCs (volatile organiccompounds) that should be avoided for environmental reasons.

SUMMARY

Thus, one aspect of the disclosure relates to a chromium-free agent forforming conversion coatings on sheet steel. Furthermore, the disclosurealso relates to a sheet steel with a chromium-free conversion coatingand a method for its production, which sheet steel can be produced asinexpensively as possible, which can be used as a replacement fortin-free sheet steel (TFS or ECCS) and tinplate, and which, especiallywith respect to corrosion resistance and adhesion for lacquers ororganic coatings, is comparable to tinplate or tin-free sheet steel.

Also disclosed are a method of producing conversion-coated sheet steeland a treatment agent for application of a conversion coating on sheetsteel.

The sheet steel disclosed, which can be, specifically, strip-shapedblackplate or sheet steel coated with an anticorrosive metal filmcoating, has a conversion coating on at least one surface, whichconversion coating contains at least one of the following components

i) hexafluorotitanate

ii) zinc phosphate and/or iron phosphate,

iii) phosphoric acid,

iv) or a mixture of components i) to iii), with the conversion coatingnot containing any organic substances.

The conversion coating preferably consists of one of the components i)to iii) or of a mixture of these components, most preferably of amixture of components i) and ii) or of a mixture of all three componentsi), ii) and iii).

To produce the sheet steel disclosed, metal-coated or uncoated sheetsteel (blackplate) is used, the surface of which is degreased in a firstprocessing step and subsequently rinsed with water or another rinsingliquid and finally, in the next step, a wet film of a conversion coatingis applied by applying a wet film of a chromium-free treatment solution,which consists exclusively of inorganic components, to at least onedegreased surface of the sheet steel, with the wet film of theconversion coating being prepared from components that were dissolved inwater and with the components being selected from the group comprisinghexafluorotitanate, zinc phosphate and/or iron phosphate, phosphoricacid and/or a mixture of these components, provided that the componentsdo not contain any organic substances and any inorganic particles withan average particle diameter greater than 0.005 μm. The wet coatingvolume is preferably in the range of 1 mL/m² to 10 mL/m². In a last stepof the method disclosed, the wet film of the conversion coating isfinally dried.

The chromium-free treatment solution preferably consists exclusively ofwater and one of the components hexafluorotitanate, zinc phosphateand/or iron phosphate or phosphoric acid, or a mixture of thesecomponents, most preferably of a mixture of the componentshexafluorotitanate and zinc phosphate or a mixture of the componentshexafluorotitanate, zinc phosphate and phosphoric acid. While the wetfilm of the conversion coating prepared from the aqueous treatmentsolution is drying, the solvent (water) evaporates, so that the driedconversion coating in these preferred embodiment examples consists ofonly the active components of the chromium-free treatment solution,i.e., of hexafluorotitanate, zinc phosphate and/or iron phosphate andphosphoric acid, or a mixture of these active components.

Preferred embodiments are also disclosed. The disclosure will beexplained in greater detail below:

DETAILED DESCRIPTION

The starting material of the sheet steel disclosed is preferablycold-rolled, annealed and temper-rolled or temper-passed sheet steelmade of steel with a carbon content of 20-1000 wt ppm. The sheet steel(blackplate) preferably has the following properties:

-   -   Tensile strength: 300-1000 MPa    -   Elongation at break: 1-40%    -   Thickness: 0.05-0.49 mm    -   Surface roughness: 0.1-1 μm

The steel of the sheet steel can be, for example, ferritic steel or amultiphase steel which comprises a plurality of structural constituents,in particular ferrite, martensite, bainite and/or retained austenite.Such multiphase steels are distinguished by a high tensile strength ofmore than 500 MPa and, at the same time, by a high elongation at breakof more than 10%. With respect to the intended use of blackplate, whichis treated as disclosed, as packaging steel, preferably the grades ofsheet steel defined in DIN EN 10202:2001: “Cold reduced tin millproducts—Electrolytic tinplate and electrolytic chromium/chromium oxidecoated steel” are used. This standard also defines, inter alia, theanalysis and mechanical properties of steel. The grades range especiallybetween TS230 (mild steel grade, batch-annealed, yield strength 230 MPa)to TH620 (continuous annealing furnace, 620 MPa). The sheet steel canalso be metal-coated sheet steel, for example, electrolyticallytin-plated sheet steel.

The method disclosed is carried out by moving the sheet steel,preferably in the form of a strip, at a strip speed of more than 200m/min and up to 750 m/min and by subjecting it to an electrochemicalpretreatment. In the course of this pretreatment, the moving sheet steelis first cleaned and degreased. Cleaning and degreasing preferably takesplace by passing the sheet steel, as a cathode, through an electrolyte.Degreasing is vital since, after recrystallization annealing, thecold-rolled and recrystallization-annealed sheet steel is, as a rule,temper-rolled or temper-passed, which, for example, during wet temperrolling, causes the surfaces of the sheet steel to be contaminated witha water-oil suspension and, during dry temper rolling, with oil, abradediron particles, soaps and other contaminants. These contaminants areremoved in the cleaning step.

To clean and degrease the sheet steel, the sheet steel can be passed,for example, through a cleaning tank containing an alkaline sodium orpotassium hydroxide solution. The concentration of the alkalinedegreasing agent is preferably in the range of 20 to 100 g/L at bathtemperatures from 20-70° C. Degreasing blackplate preferably takes placein two steps, where the first step involves a dipping process and thesecond step an electrolytic process with current densities from 2 to 30A/dm². After degreasing, each surface of the blackplate strip is rinsed,for example, through a triple cascade rinse with 10-30 m³/h water. Ifnecessary, oxide residues can be removed by passing the blackplate stripthrough additional cleaning tanks containing hydrochloric acid pickle orsulfuric acid pickle with a concentration of, for example, 10 to 120 g/Lin two consecutive dipping steps, followed by a dipping rinse in onedipping step. The temperatures of the pickling solution and the rinsingwater are typically in the range of 20° C. to 60° C.

After cleaning and degreasing, an additional electrochemical treatmentof the sheet steel can be carried out so as to create a homogeneoussteel surface which provides a good adhesion surface for a conversioncoating. In this additional electrochemical treatment, the sheet steelis used as the anode and passed through an alkaline electrolyte. Thealkaline electrolyte can be, for example, a sodium hydroxide solution ora sodium carbonate solution (Na₂CO₃).

After the anodic treatment in the alkaline electrolyte, the sheet steelis rinsed with water or another rising liquid and subsequently dried.Drying can take place, for example, in a continuous drying furnace or bymeans of a blowing unit which blows a laminar stream of hot air streamonto the surface of the moving sheet steel.

After rinsing and drying, the conversion coating is applied to at leastone surface of the sheet steel. For this purpose, a wet film of anaqueous, chromium-free treatment solution which is based exclusively oninorganic components is applied to the electrolytically pretreated anddried surface of the sheet steel. This is preferably performed in ano-rinse process in which the rinsing step after application of the wetfilm is omitted. The aqueous treatment solution which forms theconversion coating can be applied to the surface of the sheet steel, forexample, by means of a roll coater or it can be sprayed onto the surfacewith spray nozzles, e.g., with a rotation sprayer.

After application of the wet film of the treatment solution, theconversion coating thus formed is dried. For this purpose, the sheetsteel is passed, for example, through a belt dryer in order to dry thewet film of the conversion coating. Drying preferably takes place attemperatures of 50° C. to 250° C. After the conversion coating hasdried, (on each side) a dry film of the thus formed conversion coatinghaving a surface weight of 1 to 1000 mg/m², preferably of 10 mg/m² to400 mg/m², remains on the surface of the sheet steel. The desired dryfilm thickness of the conversion coating can be controlled by thequantity of the aqueous treatment solution delivered per unit time inthe application step. If necessary, the surface weight of the conversioncoating applied can also be adjusted prior to the drying step bysqueezing off any excess treatment solution.

As a final step, the surface of the dry conversion coating canoptionally be treated with dioctyl sebacate (DOS), acetyl tributylcitrate (ATBC), butyl stearate (BSO) or polyalkylene glycol, especiallypolyethylene glycol (PEG, preferably with a molecular weight of 6000g/mol), or combinations thereof.

In addition to the solvent water, the aqueous treatment solution used toapply the conversion coating to the sheet steel contains at least one ofthe following components:

-   -   i) hexafluorotitanate,    -   ii) zinc phosphate and/or iron phosphate,    -   iii) phosphoric acid,

or a mixture of components i) to iii), provided that the components i)to iii) do not contain any organic substances.

In preferred embodiment examples, the aqueous treatment solution, inaddition to the solvent water, consists of the following components ormixtures thereof (unless expressly noted otherwise, all parts andpercentages refer to parts and percentages by weight):

-   -   hexafluorotitanic acid [CAS: 17439-11-1],    -   zinc phosphate [CAS: 14485-28-0] and/or iron phosphate [CAS:        10045-86-0],    -   a mixture of hexafluorotitanic acid and zinc phosphate and/or        iron phosphate,    -   a mixture of hexafluorotitanic acid and phosphoric acid,    -   a mixture of hexafluorotitanic acid, zinc phosphate and/or iron        phosphate and phosphoric acid or    -   a mixture of zinc phosphate and/or iron phosphate and phosphoric        acid.

The effect of the treatment solutions and the properties of theconversion coatings depend on the concentration of the components usedin the aqueous treatment solution and on the wet film volume of theaqueous treatment solution on the surface of the sheet steel.

In the aqueous treatment solution, the above-mentioned components of theconversion coating can be used, for example, in the followingconcentrations, with these concentrations equally applying to anymixtures of the components among each other:

hexafluorotitanic acid: 1%, 3%, 5%, 7% and 10%, noting that these valuescan also be considered as boundaries for any conceivable concentrationranges;

zinc phosphate (Zn₃(PO₄)₂): 1%, 3%, 5%, 7% and 10% and the concentrationranges correspondingly derivable therefrom;

iron phosphate (FePO₄)): 1%, 3%, 5%, 7% and 10% and the concentrationranges correspondingly derivable therefrom;

zinc phosphate (Zn₃(PO₄)₂) and/or iron phosphate (FePO₄)) in a mixturewith phosphoric acid: concentrations same as given above for zincphosphate and iron phosphate) plus 1.4%, 2.3%, 3.2% 4.2% and 5.5%phosphoric acid and the concentration ranges correspondingly derivabletherefrom;

TiPO₄, i.e., a mixture of hexafluorotitanic acid and zinc phosphate withan addition of phosphoric acid in a ratio of 1:2.5:1 to 4:10:4 at astarting concentration of a component as specified above forhexafluorotitanic acid and zinc phosphate.

For the conversion coatings disclosed which contain titanium, i.e.,especially for the conversion coatings containing hexafluorotitanate ora conversion coating containing a mixture of hexafluorotitanate with aphosphate, the preferred dry coating weight relative to titanium wasfound to be in the range of 1 mg/m² to 50 mg/m², preferably in the rangeof 10 mg/m² to 40 mg/m². For the conversion coatings disclosed whichcontain phosphate, i.e., especially for the conversion coatings madefrom or with zinc phosphate or iron phosphate, the preferred dry coatingweight relative to phosphate ions was found to be in the range of 10mg/m² to 1000 mg/m² and preferably in the range of 100 mg/m² to 400mg/m².

Examples

In the following, embodiment examples of the disclosure will beexplained in detail. The specified concentrations of the components ofthe treatment solutions which are applied to create a conversion coatingon a surface of a sheet steel refer to the treatment solution as suchand not to possibly used starting solutions with a higher concentration.All concentrations given refer to parts by weight of the activecomponents in the aqueous treatment solution, regardless of whether theused raw materials as such had already been diluted, e.g., as aqueoussolutions.

For the tests in the embodiment examples, test sheets in the form ofblackplate sheets (uncoated, cold-rolled sheet steel) with a thicknessof 0.27 mm were used. The surface of the test sheets was first subjectedto cathodic degreasing in 5% sodium carbonate solution Na₂CO₃ (time=30s, temperature=38° C., current=5 A/dm³) and subsequently rinsed withwater and deionized water. Using a roll coater (LARA), a wet film of theaqueous treatment solutions as listed in Table 1 was applied to thedegreased surface and subsequently dried in a drying chamber (time=50 s,temperature 98° C.). The resulting surface weight of the conversioncoating (dry weight of the treatment solution) is listed in Table 1. Thesurfaces of the conversion-coated test sheets were subsequentlyinspected. The dry weight of the conversion coating was determined bymeans of XRF, and the test sheets were subjected to cyclic voltammetryto determine the electron transfer barrier. The current density wasmeasured at a potential of −770 mV. This is characteristic of theoxidation of iron into its divalent form. The higher the measured value,the greater the oxidizability. The results of the determination of thedry coating weight and of the cyclic voltammetry are listed in Table 1.

In conversion coatings containing hexafluorotitanate, the valuesmeasured by cyclic voltammetry and listed in Table 1 are relativelyhigh, which indicates that the coating is permeable. The values of theconversion coatings containing the phosphates (zinc phosphate, ironphosphate) are invariably lower, which is indicative of an opaque anddenser coating. With respect to oxidizability, the phosphate-containingconversion coatings therefore have better properties and specifically ahigher corrosion resistance. Measurements by cyclic voltammetry indicatethat the combination of hexafluorotitanate or hexafluorotitanic acidwith zinc phosphate leads to surprisingly favorable results (10 μA/cm²to 50 μA/cm²). These values are in the range of commercially availableagents with complex structures for use in the production of conversioncoatings on metals. The positive properties of the titanium-containingconversion coatings and the phosphate-containing conversion coatings canbe combined by preparing the aqueous treatment solution from a mixtureof hexafluorotitanate or hexafluorotitanic acid and zinc phosphateand/or iron phosphate with water as a solvent, in particular withpercentages by weight of 1 to 10% for the hexafluorotitanic acid and (atotal of) 1 to 10% for zinc phosphate and/or iron phosphate. Inaddition, phosphoric acid can be added to this preferred mixture, forexample, in a percentage by weight of 1 to 10%. The addition ofphosphoric acid offers the advantage that it is able to dissolve thezinc phosphate.

Subsequently, four different lacquers (Gold lacquer AN 101.597 with acoating weight of 5 g/m²; Gold lacquer BPA NI Metlac 816714 with acoating weight of 5 g/m²; Gold lacquer GL 300 MF with a coating weightof 5 g/m²; White lacquer BPA NI Valspar R 1016 with a coating weight of15 g/m²) were applied to the dried conversion coatings, and thelacquered test sheets were subjected to loading tests (deformation andsterilization) and tested using the cross-cut test and the Erichsenscale to assess the lacquer adhesion. For this purpose, a point scoringsystem was used, in which points from 0 (no adhesion) to 7 (optimumadhesion) were assigned in accordance with the quality of lacqueradhesion. The points given to a conversion coating for the differentlacquers were added together, and the sum of this addition is listed inTable 2. The higher the total value, the better the adhesiveness of theconversion coating for organic lacquers.

Table 2 indicates that the conversion coatings made withhexafluorotitanate yield the best results with respect to lacqueradhesion and that the adhesiveness increases as the coating weightincreases. For the conversion coatings with phosphates (zinc phosphate,iron phosphate), it was found that lower coating weights tend to improvethe lacquer adhesion and that, given the same coating weight, theadhesiveness of zinc phosphate is better than that of iron phosphate.Thus, zinc has a positive effect on lacquer adhesion. Zinc can bepresent in a coating weight that corresponds to any of theabove-mentioned coating weight ranges if the conversion coating containsthe component zinc phosphate. Conversion coatings with a percentage byweight of 1% to 5% of zinc (relative to the total weight of theconversion coating), preferably with 2 to 4 wt % of zinc, proved to beespecially advantageous.

The results of the lacquer adhesion tests were compared to the resultsof comparison samples made of conventional tin-free sheet steel (ECCS orTFS) and to sheet steel that had been coated with the commerciallyavailable material “Bonderite®” by Henkel. The conventional tin-freesheet steel (ECCS or TFS) was given a total score of 115 and, dependingon the coating weight, the sheet steel coated with “Bonderite®” wasgiven a total score of 88 to 118.

Based on comparison tests with different coating weights (surfaceweight) of the conversion coatings, it was possible to demonstrate thatthe conversion coating with hexafluorotitanate up to a coating weight ofapproximately 50 mg/m² (relative to titanium) yields good results. Thus,a preferred range of the surface weight of the conversion coating(relative to titanium) is in the range of 1 to 50 mg/m², more preferably3 to 40 mg/m², especially 10 to 40 mg/m² or even 20 to 40 mg/m² or 15 to30 mg/m².

With the phosphate coating, good results are obtained up to a coatingweight of approximately 500 mg/m² (relative to the phosphate ion PO₄).Thus, a preferred range of the surface weight of the conversion layerrelative to phosphate (PO₄) is in the range of 10 to 500 mg/m², morepreferably 20 to 400 mg/m², especially 50 to 300 mg/m² or even 100 to250 mg/m² or 150 to 300 mg/m².

In another test, the formability of the lacquered test sheets wasinvestigated. For this purpose, the lacquered test sheets were formedinto 13-2 cups by means of deep drawing. On the one hand, it was foundthat the conversion coatings with hexafluorotitanate (hexafluorotitanicacid) perform best, even when subjected to extreme deformations. On theother hand, from the results of formability tests, it was found that thetitanium-containing conversion coatings with coating requirements >40mg/m² are inferior, which is the reason that coating requirements ofless than 40 mg/m² are preferred for the titanium-containing conversioncoatings.

In addition, tests were conducted in which the conversion-coated testsheets were laminated with films (instead of with a lacquer coating).After deep drawing the film-laminated test sheets to form standard cups,investigations to test the detachment of the film laminate were carriedout.

For each of the treatment solutions according to the present disclosure,the test results obtained show that after a short thermalpost-treatment, the adhesion values are very good and equivalent tothose of the commercially available complex conversion agents withorganic substances.

In general, the results indicate that to create conversion coatings onsheet steel which contain organic substances (such as polymers andorganic film-forming agents) and organic compounds in the form ofparticles with particle sizes greater than 50 nm, it is not necessary touse complex treatment solutions in order to achieve good corrosionresistance and good adhesion properties for lacquers and plasticlaminates. As disclosed in the present disclosure, comparable resultsare obtained with purely inorganic treatment solutions of a simplercomposition that is limited to the active ingredients and therefore lesscostly, which are comparable to the conventional chromium-containingconversion coatings on sheet steel.

Thus, based on the present disclosure, it can be concluded that acoating with the active component titanium in the range of titaniumcoating weights of 1 mg/m² to 50 mg/m², preferably in the range of 10mg/m² to 40 mg/m², yields good results. In the phosphate-containingconversion coatings disclosed, the element zinc has a positive effect onthe properties and leads to good results up to a zinc concentration of5%.

The method disclosed can be integrated without considerable installationexpense and effort into an existing coating line, e.g., into a stripcoating line used to produce ECCS (or TFS). In such strip coating lines,the strip speed is typically 80-600 m/min.

The method disclosed has the advantage that sheet steel can be coatedwith a chromium-free conversion coating, which is based exclusively oninorganic components and which is therefore environmentally friendly,health-compatible and highly cost-effective. Sheet steel treated asdescribed in the present disclosure has excellent suitability forproducing packaging materials, especially cans, and can thereforereplace the tinplate and tin-free steel (TFS or ECCS) conventionallyused as packaging steel. Blackplate (cold-rolled sheet steel withoutmetal coating) that is coated with a conversion coating is comparable totinplate with respect to its corrosion resistance and, similarly, hasgood adhesion properties for organic lacquers and plastic coatings (forexample, of PP or PET) as tin-free sheet steel (TFS or ECCS).

TABLE 1 Cyclic voltammetry Concen- Coating (μA/cm²) tration weight 1.Cycle/ No. Active component (wt %) Solvent (mg/m²) 2. Cycle 1Hexafluorotitanic acid 1.0 Water 5.0 118.8 2 Hexafluorotitanic acid 3.0Water 12.0 93.0 3 Hexafluorotitanic acid 5.0 Water 13.0 131.4 4Hexafluorotitanic acid 7.0 Water 28.0 114.0 5 Hexafluorotitanic acid10.0 Water 39.0 154.0 6 Zinc phosphate 1.0 Water 120 12.6 Phosphoricacid (85%) 1.4 7 Zinc phosphate 3.0 Water 205 15.2 Phosphoric acid (85%)2.3 8 Zinc phosphate 5.0 Water 383 21.0 Phosphoric acid (85%) 3.2 9 Zincphosphate 7.0 Water 417 10.7 Phosphoric acid (85%) 4.1 10 Zinc phosphate10.0 Water 592 7.7 Phosphoric acid (85%) 5.5 11 Iron phosphate 1.0 Water348 24.5 Phosphoric acid (85%) 4.5 12 Iron phosphate 3.0 Water 583 18.7Phosphoric acid (85%) 5.7 13 Iron phosphate 5.0 Water 671 24.2Phosphoric acid (85%) 6.9 14 Iron phosphate 7.0 Water 968 14.5Phosphoric acid (85%) 8.1 15 Iron phosphate 10.0 Water 1153 57.6Phosphoric acid (85%) 9.9 16 Hexafluorotitanic acid 1.0 Water 52.0 Zincphosphate 2.0 Phosphoric acid 3.0 17 Hexafluorotitanic acid 1.0 Water52.4 17.2/38.0 Zinc phosphate 2.5 Phosphoric acid 1.0

TABLE 2 Concen- Coating Total score tration weight for lacquer No.Active component (wt %) Solvent (mg/m²) adhesion 1 Hexafluorotitanicacid 1.0 Water 5.0 115 2 Hexafluorotitanic acid 3.0 Water 12.0 116 3Hexafluorotitanic acid 5.0 Water 13.0 118 4 Hexafluorotitanic acid 7.0Water 28.0 120 5 Hexafluorotitanic acid 10.0 Water 39.0 130 6 Zincphosphate 1.0 Water 120 109 Phosphoric acid (85%) 1.4 7 Zinc phosphate3.0 Water 205 90 Phosphoric acid (85%) 2.3 8 Zinc phosphate 5.0 Water383 110 Phosphoric acid (85%) 3.2 9 Zinc phosphate 7.0 Water 417 65Phosphoric acid (85%) 4.1 10 Zinc phosphate 10.0 Water 592 77 Phosphoricacid (85%) 5.5 11 Iron phosphate 1.0 Water 348 88 Phosphoric acid (85%)4.5 12 Iron phosphate 3.0 Water 583 86 Phosphoric acid (85%) 5.7 13 Ironphosphate 5.0 Water 671 78 Phosphoric acid (85%) 6.9 14 Iron phosphate7.0 Water 968 98 Phosphoric acid (85%) 8.1 15 Iron phosphate 10.0 Water1153 83 Phosphoric acid (85%) 9.9 16 Hexafluorotitanic acid 1.0 Water52.0 Zinc phosphate 2.0 Phosphoric acid 3.0 17 Hexafluorotitanic acid1.0 Water 52.4 87 Zinc phosphate 2.5 Phosphoric acid 1.0

What is claimed is:
 1. A sheet steel with a conversion coating on atleast one surface of the sheet steel, wherein the conversion coatingcontains at least one of the following components: i)hexafluorotitanate, ii) zinc phosphate and/or iron phosphate, iii)phosphoric acid, provided that the components i), ii) and iii) do notcontain any organic ingredients.
 2. The sheet steel of claim 1, whereinthe conversion coating consists of one of the following components or amixture of these components: i) hexafluorotitanate, ii) zinc phosphateand/or iron phosphate, iii) phosphoric acid, provided that thecomponents i), ii) and iii) do not contain any organic ingredients. 3.The sheet steel of claim 1, wherein an organic lacquer and/or a plasticcoating is applied to the conversion coating.
 4. The sheet steel ofclaim 1, wherein the conversion coating contains phosphate ions.
 5. Thesheet steel of claim 1, wherein the coating weight of thehexafluorotitanate in the conversion coating, relative to titanium, isin the range of 1 mg/m² to 50 mg/m².
 6. The sheet steel of claim 1,wherein the coating weight of the phosphate in the conversion coating,relative to phosphate ions, is in the range of 10 mg/m² to 1000 mg/m².7. A method of producing a conversion-coated sheet steel, comprising thefollowing steps: a) having available a sheet steel, in particular in theform of a steel strip; b) cathodic degreasing of at least one surface ofthe sheet steel; c) application of a wet film of a conversion coating tothe degreased surface of the sheet steel, with the wet film of theconversion coating being prepared from components dissolved in water andwith the components being selected from the group of: i)hexafluorotitanate, ii) zinc phosphate and/or iron phosphate, iii)phosphoric acid, and iv) a mixture of any one of i) to iii), providedthat the components do not contain any organic substances; d) drying ofthe wet film of the conversion coating.
 8. The method of claim 7,wherein an organic lacquer and/or a plastic coating is applied to thedried conversion coating.
 9. The method of claim 7, wherein, aftercathodic degreasing, the degreased surface of the sheet steel isanodically polarized in an alkaline electrolyte.
 10. A treatment agentfor application of a conversion coating to sheet steel, wherein thetreatment agent contains water and at least one component selected fromthe group of i) hexafluorotitanate, ii) zinc phosphate and/or ironphosphate, iii) phosphoric acid and iv) a mixture of any one of i) toiii), provided that the components i), ii) and iii) are free of organicingredients.
 11. The treatment agent of claim 10, wherein the treatmentagent consists of water and a mixture of hexafluorotitanate, zincphosphate and/or iron phosphate and phosphoric acid.
 12. The treatmentagent of claim 10, wherein the treatment agent consists of water and oneof the following components or a mixture of these components: i)hexafluorotitanate, ii) zinc phosphate and/or iron phosphate, iii)phosphoric acid, provided that the components i), ii) and iii) do notcontain any organic ingredients.
 13. The sheet steel of claim 5, whereinthe coating weight of the hexafluorotitanate in the conversion coating,relative to titanium, is in the range of 10 mg/m² to 40 mg/m².
 14. Thesheet steel of claim 6, wherein the coating weight of the phosphate inthe conversion coating, relative to phosphate ions, is in the range of100 mg/m² to 400 mg/m².